2003 Duramax transmission issue today

[Happy Gilmore]

2003 Duramax. 120k miles. Going up vantage grade at 75mph. Truck shifts down without need or warning, locks into fourth gear wont shift out. All temps low and normal. Pulled over at ryegrass. Wont shift out of fourth. No reverse, only went into fourth. Limped it into Windy Chevy. Waiting for them to look at it. Mcdonalds big breakfast is kinda good. Haven't had one in years.

Ideas?

[jackelope]

No 2nd gear? I thought 2nd and 4th was their limp mode but it's been a while.
Check engine light?

[Crunchy]

Sounds expensive.  Hope it turns out to be a sensor or something easy to fix.

[Happy Gilmore]

It's said shift lockout or something like that. Goes into park. All other gears are like its still in park. 4th gear start was a slow roll. Got me to Chevy.

Check engine light did come on. I checked all fluids at ryegrass.

[Happy Gilmore]

Got a tech to look at it. Luckily, he said he has the same truck. Pulling a hill in OD with a small trailer uses more fluid than trans can recycle if its slightly low. This caused a code to flip the transmission into a lockout of some sort. Added two quarts, reset code and drives normal. He recommended synthetic. He said after he switched to synthetic he's never had the problem again pulling his snowmobile trailer.

[headshot5]

Nice, hopefully that takes care of it.  At least nothing is broken!   

[HntnFsh]

I think Syntec and Mobil 1 are the only fluids rated for those. TES 6 I believe.

[Woodchuck]

03 Allison trans is spec'd to take Dex III or equivalent. That said I run Amsoil multi vehicle ATF in mine and at 255k plus, have had zero issues. knock on wood. Fluid level is critical. 

[jeffro]

Keep 'er full, and use the trailer button when going up or down hills.
Had a bigger problem with a F450 when one of my drivers failed to do so.
Can't believe he got it back to the shop, but when he did, it locked up tight.

[Dhoey07]

I have the 6 speed Allison 1000 and use castrol tranSynd. Did a double drain and fill with the external filter @ 75k.

[Happy Gilmore]

This truck was serviced every 5k at Issaquah prior to my honey buying it. Impeccable service record. Just had tranny flush at evergreen chev two oil changes ago. They gave it to me for free because they didn't have the truck done on time. No leaks, it was two quarts low. Anyways, I just drove back from southern ca with a three horse trailer no issues at all. Get to Wa. Pulling the tiny little casita up the vantage grade. Seems like it would have happened with the big trailer and heavier load?

[Tikka300wsm]

You just limped the tranny. Not really a big deal, the 5 speed allisons lock in 4th. Is the truck stock or have a chip or tune on it? If you have a way to read/clear codes (ex. Gauges, chip, code reader) you just have to pull over, shut the truck off, clear the code and your good to go. Just use your tow/haul if your climbing hills even when pulling light.

[Happy Gilmore]

You just limped the tranny. Not really a big deal, the 5 speed allisons lock in 4th. Is the truck stock or have a chip or tune on it? If you have a way to read/clear codes (ex. Gauges, chip, code reader) you just have to pull over, shut the truck off, clear the code and your good to go. Just use your tow/haul if your climbing hills even when pulling light.

It's got a tuner, exhaust and pipes. I pulled over, checked fluids and turned the truck off and cycled the ignition(not sure that does anything on a Chevy but I know it will reset my ford)...

All WA fine but, still stuck in fourth

[Happy Gilmore]

03 Allison trans is spec'd to take Dex III or equivalent. That said I run Amsoil multi vehicle ATF in mine and at 255k plus, have had zero issues. knock on wood. Fluid level is critical. 

Our 2002 duramax has 265k but tranny smoked back at 200 something. My powerstroke is at 245. Never had an issue w tranny knock on wood.

[Tikka300wsm]

You just limped the tranny. Not really a big deal, the 5 speed allisons lock in 4th. Is the truck stock or have a chip or tune on it? If you have a way to read/clear codes (ex. Gauges, chip, code reader) you just have to pull over, shut the truck off, clear the code and your good to go. Just use your tow/haul if your climbing hills even when pulling light.

It's got a tuner, exhaust and pipes. I pulled over, checked fluids and turned the truck off and cycled the ignition(not sure that does anything on a Chevy but I know it will reset my ford)...

All WA fine but, still stuck in fourth

What brand of tuner? Most will allow you to read/clear codes. Cycling the ignition doesn't work on the duramax.